They're usually on a string (5)
I believe the answer is:
beads
'string' is the definition.
(beads is a kind of string)
'they're usually on a' is the wordplay.
I cannot really see how this works, but
'a' is found in the answer.
The remaining letters 'beds' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for beads that I've seen before include "Small balls for threading on string" , "Small balls with holes through the middle" .)